Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/483

Rh DRESSING.] MINING 465 Frames. The principal machines for concentrating fine sands and slimes are the frame, rotating frame, percussion frame, side-blow percussion frame, revolving belt and Frue vanner, the hand buddle, the round buddle, and the keeve. The frame is simply an inclined wooden table upon which a thin deposit is formed by the sheet of ore-and-waste-bearing water which is made to flow over it gently. The stream is then stopped and the deposit washed off by hand or automatically, and collected in pits for subsequent re treatment by similar appliances if necessary. The rotating frame is a round table with a very flat convex conical surface ; the ore for suspension flows on at one part of the centre and forms a thin deposit which is richest at the top and poorest at the bottom, and this deposit is washed off so as to form two classes by means of jets of water, under which the table passes as it turns round. Concave rotating tables, fed at the circumfer ence, are also employed. The percussion frame, the Stossheerd of the Germans, is a table suspended by four chains which receives a succession of blows from a cam in the direction of the stream flowing over it ; after each blow it bumps against a piece of timber before receiving the next blow. These bumps cause the ore to settle, and after a thick deposit is formed it is dug off with the shovel, the upper end being richer than the middle or the tail. Rittinger s side-blow percussion frame is a suspended rectangular table ABCD (fig. 101), receiving blows and bumps on the side and not on the end. A stream of orey s w water S is fed on at the corner A; clean water W is supplied by the other head-boards H, H, H ; and the table is pushed out by cams in the direction of the arrow, and is driven back by a spring so that the cross-piece strikes against a bumping-block K. The light par ticles travel down the table much faster than the heavy ones, and take a comparatively straight course ; whereas the heavy and richer particles remain on the table, subject to the influence of the side-blows, for a much longer time, and travelling along a curved path reach the bottom at F. The middle class is discharged at G and the poor waste at K. The exact degree of richness of the products can be regulated by alter- ~ ing the pointers, strips of wood ** ^1. which can be turned so as to divide the stream of ore and waste where thought most desirable. The great advantage of this machine over the old percussion frame is its continuous action. The Frue vanner (fig. 102) may be looked upon as an improved form of Bruntoii s simple revolving belt. It is an endless band of india- rubber cloth, flanged on each side, which revolves slowly in the direction of the arrows, (~J~ whilst at the same time it is ^&amp;gt;. shaken sideways by a crank motion. The orey water is fed Buddies. Fig. 102. on at A, clean water at B. The natural path of the particles is down the inclined belt, but the specifically heavier ones settle upon it and are carried upwards. Those that can resist the action of the stream of clean water at B go over the end and are washed off as the belt passes through the tank. The poor stuff falls into the waste launder. The degree of concentration can be regulated by the slope and speed of the belt and the strength of the streams of ore and water. The Frue vanner has the disadvantage that it makes only two classes, rich and poor, without any intermediate product. The hand buddle is a rectangular wooden box with a sloping bottom. A stream of fine ore and waste suspended in water is fed in at the upper end and gradually forms a deposit on the bed of the buddle. A boy with a broom keeps the top of the sediment smooth, so as to ensure regularity of action. After a thick deposit has accumulated, it is dug out in sections which decrease in richness from the upper end (1iead} to the lower end (tail). Round buddies, like rotating frames, are of two kinds, convex and concave. The convex round buddle (figs. 103 l and 104) is a circular pit with a truncated cone, or head, of varying sixe in the centre, and a bottom sloping towards the circumference. The orey stream A falling over this head runs down gently, depositing the heaviest particles near the top, the lighter ones further down, whilst the 1 Henry T. Ferguson, &quot;On the Mechanical Appliances Used for Dressing Tin and Copper Ores in Cornwall,&quot; Proc. Inst. Jfech. Eng., 1873 pi. 41. lightest of all flow away at C. The surface of the sediment is kept even by revolving brushes D. This machine may be compared to Scale. a number of hand buddies arranged radially round a centre. The deposit that is formed is dug out in rings of varying richness. Fig. 104. The concave buddle is a circular pit with the bottom sloping to wards the centre. The stream of ore is fed all round the circum ference, and runs inwards to the middle, where the lightest particles escape. The rich head is of course near the circumference. The keeve is a large tub in which the fine stuff is stirred with Keeve. water and then is allowed to settle from a state of suspension while blows are being struck on the side of the tub. The deposit is afterwards scraped out in layers which increase in richness as they approach the bottom. The series of processes employed in dressing an ore varies, Dressing not only according to the nature of the particular mineral different to be concentrated, but also according to the size of its ores - particles and the nature of the other minerals with which it is associated. With gold the reduction in size is usually effected by Gold, stone-breakers and stamps, and much of the metal is then caught by mercury ; what escapes is concentrated with its accompanying pyrites by inclined tables covered with blankets, or by buddies, and the concentrate is treated by amalgamation or chlorination. See GOLD, vol. x. p. 746. In the case of silver the ore is frequently pulverized by Silver, stamps, and the resulting pulp amalgamated in pans or barrels. The ore may also be concentrated by any of the various machines described, and delivered to the smelter. Many of the ores of silver are sent to the smelting works without any concentration by water, as this would cause a serious loss. Lead ore is generally crushed by rolls, often after a pre- Lead, liminary reduction in size by the stone-breaker. The crushed ore is classified by revolving screens down to the size of 1 mm., and the resulting grains concentrated by jigging; dredye, or grains of ore and matrix, must bu recrushed, sized, and jigged. The finer sizes are classified by pyramidal boxes and concentrated by frames, rotating tables, and buddies. Zinc ore is dressed in the same way as lead ore ; and, as zinc, galena and zinc-blende are frequently intimately associated together, it is necessary to separate them by the use of the jig, buddle, and frame. XVI. - 59